text:
If airliners profit by moving luggage from checked to carry-on, why do some LCC gate-check luggage?
According to this answer, the less luggage is transported as checked luggage, the more "room" (in terms of weight) there is to bring along cargo, and thus more profit.
At the same time, some LCCs have the practice of gate-checking a bunch of bagage, essentially moving stuff from carry-on to checked luggage. It seems to me that that too is motivated by profit. But I'm not sure how.
These two practices seem to be contradictory, since they move luggage in other directions. While I can see the profit in the former, I don't directly see it in the latter. So what is the LCC's motivation to gate-check luggage?

A number of LCCs don't carry cargo at all or have a more limited cargo business, especially in Europe and the US. This is particularly the case if they're flying smaller narrowbody aircraft on shorter routes, where freight has to be bulk loaded. Ryanair and easyJet aren't in the freight business, though Southwest and AirAsia are.
LCCs have strong motivations to reduce turnaround times and ensure maximum utilization of their aircraft. Freight can slow down operations (somebody has to get it to the gate on time, it has to be loaded/unloaded) and so can carry-on bags (everyone has to carry them, find a space for them, lift them, and once the bins are full, it takes time to collect the excess bags, tag them, and get them downstairs and into the hold). Getting passengers to gate-check bags early can help speed the boarding process. It's the gate agent's job to get the aircraft off on time, so they're motivated to gate-check bags before there's a problem fitting them in the overhead bins.
In addition, passengers find carry-on baggage convenient. You don't have to wait for it, don't have to worry about it getting lost, and you can access it during the flight. LCCs make extensive use of ancillary revenue, with a la carte airfares that involve extra charges for anything passengers might consider convenient, from a preferred seat to a beverage. As such, some airlines charge for carry-on bags (or disallow them on their cheapest fares), extracting more revenue from those who value this convenience and speeding the boarding process by reducing the number of carry-on bags.


text:
Is this scheme for a single-fan electric airship with a rocket-like geometry workable?
I am currently designing a vertical take-off and landing vehicle that uses a Tesla powertrain connected to an electric ducted fan. Please comment on if my thought process is sound on my feasibility analysis.
My initial mission goal is extremely modest to grease the wheels of my feasibility analysis and have a realistic utility in society (and for people in my sport, recreational/experimental skydiving).
The vehicle simply needs to lift itself and a 100kg payload (coincidentally the approximate mass of a skydiver with his parachute) to the altitude of 1 mile.
I think I need to calculate my total delta_v, but because I don't need a high speed (like an orbital escape velocity or something like that) I am not positive how to go about getting it without first understanding the maximum thrust from my system.
My current plan for determining thrust from the tesla-system: 1) identify maximum output of tesla battery and motor (lets say Model S P100D) 2) calculate maximum propeller size for #1 based on algorithms available in electric drone aircraft websites/forums 3) calculate potential thrust produced by #2 4) use #3 to determine if liftoff is possible carrying at least #1
My initial calculations are not intended to include all outside forces; I will incorporate these if it is in fact, theoretically feasible at all.
Based on information above as well as the weights of the battery, motor, and an estimate of a basic rocket-shaped frame, I can figure out if a basic propeller will work for this, or if I will need to incorporate an electric turbojet, which would blow up my budget for this.
It is also feasible I could have a combination of several motors / batteries attached to multiple propellers with something like a rocket sitting in the middle of an oversized quadcopter. Again, budget goes up considerably.

so why not a helicopter or airplane?

Technically it will be something in between both, likely classified as a helicopter.  I guess I don't totally understand your question.  The flight mission is to deliver a skydiver to altitude so he can jump out, then the vehicle regenerates energy on decent to prepare for another flight.  Rocket-geometry is very aerodynamic, and the physics of a rocket-shaped vehicle taking off and landing reliably have now been shown even in extremely large models (eg. SpaceX Falcon 9).

yeah an electric helicopter or airplane does all you said better and cheaper so why not? also since your velocity would be slower than a bicycle when who cares about aerodynamic?

@user3528438.  I am not sure I know what helicopter or airplane you are talking about that is cheaper.  What i'm proposing is simply a small aluminum cylinder with a salvage Tesla powertrain attached to a rotor.  Helicopters are typically quite more expensive, and there aren't a plethora of electric helicopters i know of, especially of the inexpensive type.  I reiterate that the above craft would likely be classified as a helicopter.  The airplane has many more requirements, airfield not the least of them, cost of wings and body are not cheap.  Please link me to the cheap electric helicopter.

@user3528438 also, I think you fail to understand the question.  the question is not "what is better, my new experimental theoretical rocket-shaped helicopter, or existing aircraft?"  which would be an easy answer as you've suggested.  The question is about the theoretical thought process on determining if it is even physically possible.  I'm not trying to buy the cheapest thing on the market.  This is a thought exercise in aeronautical design.  Thanks.

Delta-v is not a useful metric in atmospheric flight. 2) The amount of energy you can regenerate from descent is negligible compared to the amount of energy your aircraft used to get to that altitude.

Thank you very much @Sanchises.  I agree Delta-v not typically useful for atmospheric flight.  Can you please opine on what metrics would be best for determining some design metrics when the desired goal is to launch the aircraft (and it's payload, a skydiver) straight up 1 mile.   As for the regenerative breaking, I don't anticipate reclaiming much energy, but perhaps enough to help facilitate and stabilize a powered vertical landing.  Any constructive input is appreciated!

First, you need thrust-to-weight¹ ratio more than 1, so the device can maintain flight. You should be able to find data about how much thrust (lift in case of rotors; it's the same thing) they can produce and how much power from the engine it takes for some existing propellers and rotors.
The efficiency, that is thrust-to-power ratio, generally increases with the disc area. That is why helicopters have large rotors—they need less power to produce the desired lift.
So you'll also need enough area. You can chose a rotor like a normal helicopter, or you can choose multiple propellers in a style similar to

(via this answer)
or

(from this company)
You should be able to get (static) thrust and efficiency for some standard propellers to use in your estimates.
Keep in mind, that the maximum thrust decreases with air density and you'll need the thrust up to your desired altitude. At 5000 ft, the density is ~86% of that at sea level, further subject to weather as density also decreases with temperature.
Now when you have a set of propellers that can produce enough thrust with your available power, you'll still need some excess power. This excess power will roughly indicate your maximum climb rate: just divide the excess power by weight².
Note that the excess power will decrease with altitude as the efficiency will decline with density.
And last you'll need to make sure you have enough energy stored to maintain full power for the time it will take you to climb to that altitude.

¹ Weight is a force acting on a body due to gravity, i.e. mass times gravity constant, so it can be directly compared to other forces (thrust).
² Since weight is force, dividing power by it yields (vertical) velocity.

Note regarding delta_v. That is a characteristic quantity for rockets, where the limiting factor is the momentum they can impart to the exhaust gasses that are generated from the fuel only. And since momentum is mass times velocity, dividing the momentum by mass gives a useful characteristic value, the $\Delta V$, with dimension of velocity.
However you have propellers and they impart the momentum to surrounding air, which you have unlimited supply of. So the momentum is not limiting for you, the energy that you lose with it is. You minimize the energy by using a lot of the air, i.e. increasing the mass of the reactive mass, not its velocity. Rockets can't increase the mass, as the fuel is all they have, so they must maximize the exhaust velocity instead and are intentionally inefficient in the sense of the thrust-to-power ratio mentioned above.

Wow; Thank you a ton Jan.  You've articulated that so incredibly well and boiled down a complicated concept into a simple thought process.  I can't thank you enough.  I think I have enough design constraints and component parameters to put together some realistic estimates.   Thanks again.

The best propeller driven vertical take-off and landing craft would be a helicopter, actually. Yes a duct around a propeller increases thrust, but upsizing a propeller creates way more thrust from the available power.
Propeller thrust has scaling effects with size due to different Reynold numbers, data does not scale up well. Best way forward from where you are now would be to look at the weight, rotor dimensions and installed power of existing designs for small helicopters such as the R-22. Or better yet, design an electrical version of the Hiller Flying Platform Platform.

Great point!  I am pretty much proposing a Hiller Flying Platform shielded in a rocket-body (to protect from the elements and normalize aerodynamic forces) and adding basic self-balancing flight-controller dynamics to negate the need for a pilot.  Given that the Hiller Platform works (worked) with 2x 44HP motors, I am pretty confident the 700+HP Tesla dual-motor could produce the necessary lift to even carry it's heavy 1,200lb battery-pack (of which really only a fraction would be required to bring the craft to 1 mi. altitude...

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
Is the FAA's International Flight Information Manual available for download? If so, from where?
This FAA website indicates that the "International Flight Information Manual (IFIM)" is a downloadable publication of the FAA.
I've never heard of this manual before today, and would like to download a copy of it.  However, clicking the provided link takes me to this page, which doesn't seem to mention the IFIM at all.
Have I missed spotting it?  Is it available for download from somewhere else?  Or is this a mistake or outdated information?

I can't find an International Flight Information Manual, but there used to be an International Flight Information Manager (IFIM), which was discontinued on November 14, 2014. Here is an archive.org link mentioning its discontinuation. It seems likely that this is the same document.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Why doesn't frost form on a moving airplane?
When I was a young student pilot, long before the FAA prohibition on polished frost went into effect, an instructor left a six-inch square of frost on the wing to demonstrate how fast it would disappear after takeoff.  Sure enough, we hadn't even reached pattern altitude and the frost was completely gone.  Furthermore, in my experience frost never forms on an airplane as long as it's moving, or as long as there's sufficient airflow moving across it.
I've never been able to find a satisfactory explanation for this.  Why doesn't frost form on a moving airplane?

After your takeoff, had the ambient air temperature plus the ram air temperature rise been less than freezing, the frost would not have disappeared quickly. It would still have sublimated eventually, but that would have taken more than a few minutes.

Dry air will make frost disappear through sublimation. However moist air will create frost on the plane.

I've had my wings ice up in VMC at night - very scary!

Frost can be formed in two ways. On a colder object by water directly desublimating on its surface or on any object when air is saturated with water.
Frost can form on a moving airplane, but only the second way. Why would they have anti-ice on pitot tubes, propeller and leading edges otherwise?
When the air is not saturated with water, ice will sublimate. If the air is moving it carries away the sublimated vapour and thus increases the sublimation rate. When flying in visual meteorological conditions, that's generally the case, so frost won't form in VMC, thogh watch out when dew point is close to temperature; in such case light haze may form that already causes some icing while visibility is still above the VMC limit. Ram and friction heating also help sublimating the ice a bit.
However when you fly into a cloud in freezing temperature, the water droplets will freeze on the aircraft when they hit it. They will hit it on forward-facing surfaces: wing and stabilizer leading edges, propellers, engine inlets, pitot tubes and nose and windscreen. That's why these components have anti-ice, either using heating or deicing boots.
VMC-only aircraft will often only have pitot tube heating and carburettor heating, because those can sometimes gather frost in sufficiently moist air that is not yet forming cloud. This lack of anti-ice makes flying into IMC even more dangerous in winter than it already is for all the other reasons.
Icing only happens when the cloud contains supercooled water droplets, which occurs in clouds where the ambient air temperature is between freezing point (0°C/32°F) and around -40°C/-40°F when the droplets freeze to ice pellets. The exact point the droplets freeze varies somewhat because it depends on other parameters like amount of dust, but pilots can check whether they are accreting any ice by looking in the corners of the windscreen. Whether freezing conditions are to be expected will also be mentioned in the detailed weather forecast.

I agree with almost everything you've said but i want to make sure we're talking about the same thing. If frost won't form in VMC, why will an airplane sitting on the ramp on a clear calm winter night quickly frost up?

@SteveV.: Because as temperature decreases in the night the temperature gets slightly below dew point, light fog forms and starts to condense on things as dew and freeze. True, the fog only needs to be very light for this, so the conditions could still qualify as VMC.

You'll find supercooled liquid drops anywhere between the 0 C and -40 C levels.  Above the -40 C level you'll still fund liquid where there are very strong updrafts.

In flight, this would be known as icing and is formed in a different way (moisture deposited as a liquid which then freezes, rather than desublimating directly from water vapor to frost).

So how can frost form during flight - due to the pressure loss of the air flowing around the airplane,  when the air is saturated it will condense and instantenously freeze?

@Konrad a bit can form that way. but usually the water is already condensed and supercooled (cloud, freezing temperature) and freezes on contact because the surface becomes a nucleation site.

Frost does form if you fly into cloud with the correct temperature and moisture levels as explained elsewhere. It can bring planes down if not protected by anti ice systems or where such systems are not used correctly.
Secondly, be wary of taking off with frost on your aircraft. It could have unknown effects on the stall speed and aerodynamic qualities of your aircraft (in other words it might not fly). For example if you have frost on one wing only, or the tail, the outcome might be very bad.
Finally, faster aircraft such as jets have higher skin temperatures on the leading edges due to friction from the speed. They still need anti icing systems though. Taking to extremes the Lockheed SR-71 Blackbird had skin temperatures of 200 degrees C + when flying at Mach 3. It would also be above most of the clouds that could form ice at that point (not sure of the situation though when it was at 30,000 feet refuelling).

A parked aircraft on a still, clear night radiates its heat into the dark sky and becomes colder than the surrounding air. If it drops below the dew point, dew forms; if it drops below that and freezing, frost forms.
In flight, the high speed air flow over the aircraft keeps its skin at essentially the same temperature as the air so it doesn't drop in temperature the same way.

On a still, clear night, the plane will be in thermal equilibrium with the atmosphere, i.e., at the same temperature as it. If the plane somehow got colder, the atmosphere would conduct heat to it and warm it up again.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is there a way to register for updates on certain NTSB investigations?
I would like to track the progress of an investigation of a particular accident, NTSB WPR18FA200.
Some areas of the FAA website (advisory circulars, for example) allow users to register to receive updates by email when these items change.  Is there a similar functionality available for users of the NTSB accident database?  If so, where do I find it?

I've never seen a way to do that, but agree it would be a very useful feature to have.

You could register the case number with a Google alert. Places like Kathryn’s Report usually report on the final NTSB results when they come out and if it is big enough, other sited do so as well.

No, the NTSB does not have such a service on their web site. In theory you could track the progress by downloading their database and parsing it to determine if status has changed, or by figuring out how their query system can be invoked, but those options require programming skills.


text:
If a GPS displays the correct time, can I trust the calculated position?
Suppose I plan to reactivate an airplane that has been stored in a boneyard for a year or two, or at least power it up and turn on the avionics.  Also suppose for the sake of the question that the GPS does not remember its last position and needs to recalculate it.  Finally, suppose that I don't know precisely where the aircraft is (the boneyard is a featureless place) but I do have access to the precise time (via WWV HF broadcasts or other means).
When I turn the airplane on and the GPS starts receiving satellites, it will begin to calculate its own position.  It will also determine the current time. Time and position are mutually necessary to get a GPS solution.
Therefore, though I have no ability to verify my actual position, if I verify the GPS has calculated the correct time, can I be sure my calculated position is correct?
Phrased another way, "is calculated GPS time a proxy for HDOP?"

I hesitated before asking this question here - it's asking more about GPS in general than about GPS as used in aviation, and it might be a better fit on GIS.  If a consensus emerges, I'll move the question there.

How accurately do you know the time is correct? If you know it's within a second, then you know the position is within 300,000 km...

Why not just compare it with what your phone says?

Well there was this one time where a CEO spoofed the position in order to start a major war so that his news corporation could cover it, but someone named j. bond put an end to that

WWV broadcasts the correct time at the transmitter. To know exactly what time it is where you are, you need to know how far away the WWV transmitter is and subtract the propagation time. But since you don't know where you are, WWV is no help. You'd have to carry your own atomic clock to the boneyard. Slowly.

GPS works on time differences. Two satellites tell you the time, three give you a position assuming you are at a known height on the earths surface, four give you a 3D fix in space, 5 give you an accuracy check on that.
Nowadays, you have many sats visible assumimg a clear view of the sky.
When GPS was starting, (specialist) ships would carry atomic clocks.

How old is the receiver GPSS can only count up to 1024 weeks(about 20 years) and rolls over which creates some problems with the date and time. Aviation GPS receivers normally have a signal status page that shows RAIM prediction, WAAS availability, individual satellites currently in contact and the quality of the contact. As some say, it helps to RTFM.

Regarding the aforementioned week count rollover, the position can actually be correct even if the time displayed is completely wrong (by those 1024 weeks). This happens for my handheld Garmin GPS receiver.

In general, if the time is correct, then the position is also correct. But the time needs to be correct to an accuracy of about 10 nanoseconds.  You probably don't have access to an atomic clock, and what you see on the display is already delayed by miliseconds anyway,  so there is really no way to proof that the time is correct to the appropriate level.
HDOP (Horizontal Dillution of Precision) and TDOP (Time Dillution of Precision) are related, but don't say all too much about the actually achieved level of correctness. They are derived from the geometry of the satellite constellation as seen by the GPS receiver from your (calculated) position. Position accuracy reported by the GPS receiver is based on HDOP, under the assumption that the GPS system is functioning nominally.
What you need to look at is the Horizontal Protection Limit (HPL) or Horizontal Integrity Limit (HIL), also called containment bound. This is the current maximum position error that the GPS can output in case of a GPS system fault (e.g. due to data corruption in the GPS almanac or GPS satellite failure). If the HPL/HIL is below 0.2 NM, you should be good.

Having sat for 2 years, the GPS receiver will need to get the full almanac, which takes time. While this is happening, it won't "know" its position at all, but wouldn't it have a time value accurate to better than one second (i.e. what it displays would look like the correct time)? Or is the conversion from GPS time to GMT also contained in the almanac?

The GPS - GMT offset is contained in the almanac. But as soon as the receiver has the offset, it theoretically can produce a time with an accuracy of less 1 second, without knowing position.  It doesn't need the whole almanac, the assumption that the receiver is near earth should give you an accuracy down to a few centiseconds.

I think the fact that you are in line of sight to any GPS satellite tells you the distance to that satellite is between about 22,000 and 28,000 km. By assuming the distance is 25,000 km the time sent by the satellite tells you your time within 10 milliseconds (3,000 km divided by speed of light). So once you get the almanac you know the position of each satellite within 10ms which will be better than 100 meters.

The WWV transmitter does have access to an atomic clock, but it only guarantees 100 ns of accuracy relative to UTC.

@Mark that is at the transmitter. But you don't know where the receiver is located and what the atmospheric and ionospheric propagation conditions are, so the received timestamp is far less accurate.

If the GPS complies with the TSO, it will either not output a position (in ARINC, SSM=NCD) or will flag the output as failed (SSM=FW) if it does not meet requirements. The only caveat would be that it could output valid position (SSM=NO) and also set a RAIM alert. This would be likely during a cold start as it acquired satellites, first with just enough to compute a position but insufficient to complete the RAIM checks. If there's valid data and no RAIM alert, the position is good enough for use. HPL/HIL will tell you how good.

If the time is correct, the position is likely (but not necessarily) correct.
Consider the extreme example where your GPS device can only acquire one satellite. You would have a good idea of the time, but with just one satellite it is impossible to even get a rough estimate on your position.

But how do you know that the time is correct? A radiocode clock is entirely at the mercy of multipath effects etc., and consumer-grade ones are focused on decoding the time digits from the signal rather than synchronising the decoded time to the incoming leading edge. Even the PPS output from consumer devices can be wildly out- 10s of mSec- when compared with a precision source (aggregate of multiple GNSS constellations, or an atomic clock transported, as somebody has already said, slowly). In short, you can't trust anything from GPS until the receiver says it can be trusted.

Is it possible to have an ongoing fault that affects position but not time?  Yes.  I saw a noncompliant antenna installation once that made position jump.
Is it going to be accurate 100% of the time if it has accurate time?  No. There are satellite failures, jamming, ionospheric interference, and multi-path interference.  Horizontal Integrity Limit, also known as HPL, gives a good look at the possible impact of those and will increase when the accuracy is degraded, including when your accuracy depends heavily on a single satellite.
Can you use it for regulations that require accurate GPS like ADS-B transmission or RNP navigation?  Unfortunately probably not until a certified repair shop looks at it for return to service.
Typically if you're unsure of the hardware, throw in another unit without the same possible faults so you can compare the two.

It is a very interesting question and subtly different than the much more commonly asked question which is how accurate is GPS, or what method provides the greatest GPS accuracy. Your question instead is about verifying that the location result that your GPS receiver has calculated is accurate.
To answer the basic question at least in theory, the answer is yes because a GPS receiver calculates position and time simultaneously as it solves the (x, y, z, t) problem. So if you are able to verify to the nanosecond that your GPS receiver has correctly calculated the time, then the position will be accurate also. A corollary question would be is it possible for a GPS receiver to calculate the correct time while meanwhile calculating the wrong position. I don’t know how that would be possible so I would say the answer to that question is no.
The challenge however is how to implement your solution of using time to verify the location accuracy. Your WWV example introduces a practical aspect to your question, although I don’t know if that was the intention of using that example. The constraint on the solution depends on what definition is used for practical.  At the far extreme of practicality would be to use an atomic clock to verify the accuracy. However just purchasing or renting an atomic clock would not be enough as you will need some type of interface between the atomic clock and the GPS receiver that can compare the time difference between the two and display the result.
However even an atomic clock solution will still probably only be able to verify location accuracy within about five meters or so. This is because the GPS system uses GPS time which is not based on the Earth’s rotation like UTC is, so it does not include leap seconds. That in itself is not a problem because the satellites broadcast this difference in whole seconds so that GPS receivers can make the adjustment (I think the current adjustment is around 10 seconds). The problem is that GPS time is maintained separately from UTC time, and there is a process of “steering” GPS time within the GPS system to align within whole seconds of UTC. But this steering process is estimated to be accurate to only within about 14 nanoseconds (1).  Light travels one meter in three nanoseconds so the discrepancy between GPS time and UTC would create a potential error of about five meters if using a UTC based atomic clock to verify if your GPS location is accurate. However if you are able to obtain an atomic clock calibrated to GPS time then you could verify position accuracy at the submillimeter level, which certainly will be accurate enough for whatever latitude/longitude decimal precision that your GPS unit outputs.
If there is a constraint that requires using a method less elaborate than using an atomic clock, then the ability to verify location accuracy is going to decrease dramatically. Using WWV for example will be much less accurate since WWV is only accurate to 100 nanoseconds, or about 30 meters (2). And even that level of accuracy requires that you somehow know your exact distance from the WWV transmitter, which is something of a catch-22 since in fact your exact location is what you are attempting to use UTC time to verify. In other words if you used the GPS location output to determine the distance to the WWV transmitter, then if the location is inaccurate this will give you an incorrect WWV time.
If you attempted to use something like Network Time Protocol (NTP) to obtain the exact time, that is only accurate within tens of milliseconds, which would not even help you verify that your GPS has you in the correct part of the country.
Although it is outside the scope of your question, the only practical solution is to use other methods to verify your location instead of using time. Using other methods you can get at least as much accuracy as the 5 meter accuracy that you would get using a UTC atomic clock. For example you could use a high-end geodetic grade GPS receiver. These will be in the several thousand dollar range, although presumably far less expensive than renting an atomic clock. These are typically referred to as GNSS systems (global navigation satellite system) because they are multi-frequency receivers which use more than one of the available satellite systems, GPS, Russia's GLONASS and Europe's Galileo systems. And you would want to use a high-end GPS antenna to help eliminate multipath reflection (like from other planes in the boneyard).
However just using a high-end GPS receiver still requires having trust in the equipment, which does not meet the spirit of what your question is asking, which is verification. A better solution to meet your criteria of verification, as well as achieve maximum accuracy would be to use an RTK system (real-time kinematic positioning) similar to what is used by surveyors. With this type of system you would place a base station transmitter at a precisely known location, such as the roof of the hotel where you are staying, or maybe a nearby gas station if they are amenable to this. The base station needs to be located no more than about 10 miles from the boneyard. At the boneyard you would have a mobile unit which communicates with the base station via UHF. The two stations compare the satellite signals that they are receiving and the mobile unit uses that information to make corrections and further refine the location accuracy, in some cases to within centimeters if you let the process run long enough as it will gain accuracy over time.
The reliability of RTK systems is possibly due at least in part from using the principles of Common View GPS Time Transfer (3).
(1) GPS time vs. UTC gpsinformation.net
(2) How accurate is WWV time www.nist.gov
(3) Common View GPS Time Transfer tf.nist.gov

RTK stations
(Author: TS Eriksson via Wikimedia Commons, CC BY-SA 4.0)

RTK mobile station
(Author: SparkFun Electronics source: www.flickr.com via Wikimedia Commons, CC BY-SA 2.0)

All of this is certainly true but I'm confused about how it answers my question regarding time.

I may have misread your question as having two aspects. The first aspect is straightforward, if you obtain accurate time independently, can you use this to verify that your GPS receiver is providing an accurate location. However your statement about using time as a proxy of HDOP caused me to perhaps inadvertently assume that you were also asking about using accurate time to obtain an accurate GPS position, i.e. using accurate time alone to correct for errors. This is what my answer was based on. My examples were meant as illustrations of why this is not the case.

Also using WWV as an example instead of using an atomic clock made me think that you are looking for a practical solution, not just theoretical (even though WWV is not really feasible). Looking at other answers and comments it seems that others interpreted the question this way also, as there are other "non-time" based suggestions given. If your question is limited to just using time to verify that the calculated position is accurate, either theoretically, or practically, or let's say with the constraint of using an atomic clock being the limit of practicality, then I will revise my answer.

I have revised my answer to hopefully better reflect the intention of your question.

It sure does, thank you!

@Mins - I don't see how it's possible to receive a signal from just one satellite and calculate GPS time. Without knowing exactly where you are you have no way to determine how long it took the signal to get there. As for x,y,z,t my assumption is that t must be calculated in order to calculate GPS time. And t cannot be calculated separately from x,y,z.  Unless you are saying that it is possible using three satellites to calculate the time difference between the receiver and the transmitter without simultaneously calculating position. The additional info includes things like known system errors

@StevePemberton: The exact time of the transmission is provided by the satellite. Of course there is a travel time, but what you may forgot is GPS doesn't calculate actual ranges, but pseudoranges, the difference allows the GPS to work. The exact time of reception is not required, the only requirement is all times of reception (4 satellites, 4 reception times) must be affected by the same time error (receiver clock error/offset) t. See how this offset is computed (as d) here.

@Mins – I think you are going into details that are not being debated. It is correct accurate receiver time is not needed at the beginning, in fact it is what the receiver software will be calculating along with receiver location using the data from multiple satellites. Pseudorange is the initial range estimate which is based on the current (i.e wrong) receiver time. Then using signals from four or more satellites the receiver simultaneously calculates the true GPS receiver position and GPS time. None of this makes it possible to determine accurate GPS time from just one satellite.

@StevePemberton, I think you're right, to determine the GPS time of arrival with only one satellite you must know your current position, which is not the assumption in the question (but some GPS synchronized clocks can work with a single satellite, provided the position is known).

authors: Vladimir F Героям слава


text:
Is there a designated air-to-air HF frequency in the USA?
Suppose I'm flying somewhere over the middle of the United States, happily telling stories exchanging important flight-related information with another aircraft on 122.75, the designated VHF air-to-air frequency.
Now let's say that the other aircraft has reached the limit of VHF transmission range, and I still have more to say.  We're both HF equipped.
Is there a designated HF frequency available for air-to-air use in the USA?

If you're telling stories, the amateur radio service is intended for that, has significant swaths of HF spectrum, and airborne use is permitted in the US. (Previous question)

Reviewing 47 CFR 87.187 - FCC, Safety & Special Radio Services, Aviation Services, Aircraft Stations, Frequencies, it does not appear that the FCC has designated a dedicated air-to-air HF frequency. All the air-to-air frequencies in the regulation are in the VHF range. Sections bb through hh.


text:
How is a flight routed from the last waypoint of a STAR to final?
If you look at a flight plan on FlightAware, the last code will be a STAR. Take a flight into JFK, for example, that uses LENDY6: the last waypoint of LENDY6 is the LGA VOR/DME at FL190. If the plane then lands on 04R, how does it get routed to final? Are there any documents that show this routing?

Look more carefully; says it right at the bottom. At the STAR's termination waypoint, LGA, expect radar vectors to final.
If you had comm failure, and couldn't talk to ATC, you would fly the STAR in your clearance and if it is an open STAR like that, at the termination waypoint you would find your own way to the ILS/LOC and shoot the approach, taking the most logical path to get there and avoiding any unusual maneuvers or turns.
A lot of the procedures you see in SIDs and STARs are not followed exactly as depicted when ATC is giving instructions (the controller may put you on vectors long before you reach the termination waypoint for example), but are there in case of a communications failure, to ensure that an arriving or departing aircraft has obstacle clearance and so that ATC can predict with reasonable certainty where you are going to go as you arrive with no way to talk to them.

Just to add to your answer, some airports have STARs waypoints all the way to the Localiser, such as Melbourne Airport YMML (e.g. Arbey One Alpha Arrival (RNAV)).

Yes that's a closed STAR. I should've mentioned the difference.


text:
If the push was accidentally started with the brakes on, what will happen since these towbars don't have shearpins?
The push was started accidentally before the brakes was released. What will happen since it doesnt have shearpins?

While it isn't desirable to try to push an airplane with the brakes set, nothing much happens if you try. Nothing moves, and the tow bar is so strong in straight compression/tension that it isn't likely to be damaged.
Problems are much more likely once momentum and moving tugs/airplanes get involved; that is when a sudden brake application is more likely to "break a towbar" (actually, a shear pin or coupling -- designed to let go before you damage the aircraft landing gear). But even that is a side-to-side failure mode, rather than pure compression.
To answer the edit to the question about "without shear pins": pushing straight back, same, nothing happens. If the aircraft applies brakes while being turned and the towbar can't break, then either the tug gets slammed to a halt, or else something gets damaged - probably at or near the point where the towbar connects to the aircraft. (Or possibly both happen.) That's bad... aircraft out of service, seriously expensive repairs, etc. Which is why shear pins ARE used in towbars. Repairing damaged towbars is far cheaper that repairing damaged aircraft.


text:
Trying to find air crash for a case study
trying to find a case study of an air crash that has some factor to do with generational differences.
I remember once crash where the FO just watch the aircraft go below G/S and crash and didn't say anything to not offend the captain.
Any help finding this crash or anything like it?

What about the Tenerife accident?

Could it be https://en.wikipedia.org/wiki/Alitalia_Flight_404 ? In the English Wiki it is a bit less explicit, but the German wiki mentions how the conflict between the experienced pilot (47 years old) and the inexperienced FO (28 years old) caused the warnings from the FO to be ignored by the captain.

It's usually not phrased as "generational differences" but if you look around for crashes where poor crew resource management ("CRM") is noted as a cause, you should be on the right track to find this kind of accident report, especially ones that talk about a steep "authority gradient" (meaning the pilot in command is extremely senior/respected and domineering over a copilot who feels he can't speak up or call out the PIC's mistakes).

So, you have hypothesis but cannot find any proof supporting it? Maybe your hypotesis is wrong if you have to search with little luck and you are only getting far-fetched evidences?
Most of the list is CRM, not 'generation' driven. Like in case of that Korean flight - both pilots were same age, what was the root cause was not about generational differences.
&

That can be the accident of Korean Air 801 in 1997. However the captain was 42, the first officer 40 and the flight engineer 57.
The captain with a high reputation gained while landing an aircraft with an engine failure at low altitude was "Pilot Flying". He was approaching Guam airport runway that had its ILS out. The ILS receiver picked a wrong unrelated electronic signal on the ILS frequency. This signal guided the aircraft on the wrong approach path.
The first officer and the flight engineer realized the slope was obviously too steep to be the correct glidepath, the runway was not in sight, and heard the ground proximity warning, but none of them did dare to stop the captain. The aircraft crashed killing most of the people onboard.
The accident investigation noticed the captain error, but also the lack of appropriate reaction from the other crew members. This accident, following other ones like the Tenerife deadly collision, led to improve Crew Resource Management (CRM). CRM is aimed to create a the appropriate interpersonal relationship "for gaining and maintaining situational awareness, for solving problems and for making decisions."
From the NTSB report:

The first officer and flight engineer noted the ground proximity warning system (GPWS) callouts and the first officer properly called for a missed approach, but the captain’s failure to react properly to the GPWS minimums callout and the direct challenge from the first officer precluded action that might have prevented the accident.

The first officer and flight engineer failed to properly monitor and/or challenge the captain’s performance, which was causal to the accident.

The FAA also stated that, on February 25, 1997, it issued a revision (Change 2) to AC-120-51B [...] that the training should emphasize prevention through effective communication and decision behavior and the importance of immediate, decisive, and correct response to a ground proximity warning.

According to the FAA, paragraph 2(m) reiterates that training should emphasize the importance of recognizing each pilot’s relative experience level, experience in specific duty positions, preparation level, planning level, normal communication style and level, overload state, and fatigue state.

On a more general standpoint, these accidents are relative to a non assertive action from the pilot not flying:

Jetstream into Hibbing, MN, (NTSB, 1994a);
DC-8 into Jeddah, Saudi Arabia (NTSB, 1993a);
C99 into Anniston, AL, (NTSB, 1993b);
Beechjet into Rome, GA, (NTSB, 1992a);
DC-8 loss of control at Toledo, OH, (NTSB, 1992b);
707 fuel exhaustion into JFK, Washington DC, (NTSB, 1991);
L-1011 windshear accident, D/FW Airport, TX, (NTSB, 1986);
MS-748 electrical failure in Pinckneyville, IL (NTSB, 1985);
737 out of Washington National, Washington DC, (NTSB, 1982);
DC-8 fuel exhaustion in Portland, OR, (NTSB, 1979);
727 into Dulles New York City, NY, (NTSB, 1975);
DC-8 freighter into Cold Bay, AK, (NTSB, 1974);
Convair into New Haven, CT, (NTSB, 1972);
L-188 into a thunderstorm at Dawson, TX, (NTSB, 1969);
Lear Jet out of Palm Springs, CA, (NTSB, 1967);
F-27 into Las Vegas, NV, (CAB, 1965).

From TO INTERVENE OR NOT TO INTERVENE? THE CO-PILOT'S CATCH 22

East Asian crews are notorious for not daring to question the captain's decisions, but this is a cultural issue rather than a generational one.


text:
Using the *liters* marks on E6B
See the attached picture. It is an Aero Products Research E6B Model 4. Notice the liters marks under imp. gal. and u.s. gal. How can I use the liters marks? Can I convert quantities to and from liters?

I reviewed the instructions manual. It is for a variety of E6B models and does not cover this small detail of having liters under imp. gal. or u.s. gal..

Much prefer the Jepps circular computers. They don't have that slidey bit so the smaller one fits in your shirt pocket, you can use them single handed, and with a bit of practice you can do wind problems just by eyeballing it with decent accuracy.

I may be wrong as I have a different model of E6B, which does conversions differently from yours, but based on this larger picture

It looks like you line up the black marker (on the "10" on the inner scale) with the conversion you want to do (eg "U.S. gal / liters"), and read US Gals on the outer scale and Liters on the inner. A good check is that 10 US Gallons should be about 37.9 litres.


text:
What priviliges and limitations does an Aircraft(General) mechanic have versus an A&P?
The FAA provides practical test standards for Airframe, Powerplant, and General mechanic certificates. Mechanics seem to demonstrate a preference for Airframe & Powerplant, so much that "A&P" is common shorthand for "aircraft mechanic".
That being the case, my questions are:

What can a mechanic who is only "general"-certified do and not do, compared to an A&P?
Is there any real-world benefit in holding a general mechanic certificate?

None. It is required to pass your written general to move on to obtain your Aiframe/Powerplant certification via an Oral & Practical exam. There is no general certification.
If you're looking for a way to work without certification, you can work under a 145's certificate without any.

Interesting! I wonder why they chose to do it that way. Thanks!


text:
What does "when used as an alternate" mean in the context of 135.225(e)?
Here's the text of 135.225(e)

[No pilot may begin an instrument approach procedure to an airport unless] The MDA or DA/DH and visibility landing minimums prescribed in part 97 of this chapter or in the operator's operations specifications are increased by 100 feet and 1/2 mile respectively, but not to exceed the ceiling and visibility minimums for that airport when used as an alternate airport, for each pilot in command of a turbine-powered airplane who has not served at least 100 hours as pilot in command in that type of airplane.

I'm interested in the bold part of the text.  Does it mean:

If a pilot shoots an approach at their destination, misses, and chooses to go to their alternate, they need not increase mins at the alternate
If a pilot intends to shoot an approach at their destination, they need not increase mins if doing so would make those mins higher than if the airport was their alternate
Or something else entirely?

References for your answers are appreciated!

The "No pilot may begin an instrument approach procedure to an airport unless" is a section of it's own, named (a), followed by conditions (1) and (2). Section (a) is not linked to section (e) in the manner you suggest.
Section (e) deals with a

... pilot in command of a turbine-powered airplane who has not served at least 100 hours as pilot in command in that type of airplane.

The portion you emphasized makes an exception to the rules set by section (e) in the case where the approach is made to an alternate airport.
So simplified version would be that a pilot with less than 100h of PIC experience in turbine aircraft must use higher minimums when flying IFR approach in a turbine aircraft. If the approach is being made to an alternate airport, the minimums of that airport are used.

As a sidenote, I firmly believe that regulatory texts in aviation are deliberately written in the most obscure manner possible. I guess that explains why I know so many lawyers that fly, they must love this s__t.


text:
What factors cause a turbine aircraft to have decreasing fuel burn over time?
Consider a fictional aircraft with a fuel schedule similar to the following:
1st hour - 10,000 (lb or kg, doesn't matter)
2nd hour  - 8,000
3rd hour  - 6,000
4th* hour - 5,000
 * and subsequent

As we can see the fourth hour of flight burns only half the fuel of the first hour, so fuel consumption is a function of time.  Is the decreasing weight of the aircraft as fuel is burned the only reason for this increase in fuel efficiency, or are there other influences?  For example if a jet launches with half tanks versus full tanks, would you use the same 1st hour fuel schedule?
I may be way off base here, so if there are fundamental errors in my question, please correct me.

@ymb1 - Very relevant - I'll spend some time reading your answer there (and possibly mark this question as dupe later).  Thanks!

Those numbers seem a bit exaggerated to me, but in any case, it's mostly due to the power reduction to keep a constant cruise speed as fuel is burned off, plus the extra burn of the takeoff and climb at the start. My own little plane is about 7 mph faster solo with minimal fuel compared to at gross, and that would represent a noticeable reduction on power to maintain the same speed.

The reduction in fuel consumption over time also applies to piston aircraft. Piston aircraft are generally more fuel efficient, so the effect is less prominent.

@John K: While the fuel consumption/speed increase isn't all that noticable (at least to me) in small piston aircraft, where you really see an effect is in rate of climb. (And takeoff distance.)

Per this paper:

Fuel consumption increases (approximately) linearly with aircraft weight, because weight corresponds to required thrust and thrust corresponds to fuel consumption.

The paper lists other fuel consumption factors as well.  But, as you suspected, aircraft weight is a dominant factor.


text:
What are the Boeing 777 thrust calculations?
Can someone show typical calculations for a Boeing 777 jet engine for the following criteria:

Take off thrust of the engine at 170 mph take off speed and 45 degrees F
Cruising thrust required at 500 mph at 35,000 feet altitude?

This might be too broad as takeoff thrust especially varies depending on runway length, takeoff weight, elevation and ambient conditions.

@Ben, thrust is not depending on runway length. I really can't see how that could be possible

@neville, this looks a bit like a homework question. could you please show us what you have tried and why can't you answer?

@Federico Derated takeoff - where less than maximum thrust is used for takeoff when it is not needed, to protect engine wear. Airbus uses a similar process called Flex Temp.


text:
What´s the logic behind the Power Curve?
Aircaft thrust is what counteracts drag, but in flight school they tought us that propeller aircraft are "power producers" and not thrust producers as jets. (That´s because the propeller needs the power of the engine to rotate and generate thrust, while a jet engine generate it by itself).
In both cases we can look at the thrust curve to get some performance information, but for props, we need to get a power curve also (get it multiplying thrust times airspeed values of the thrust curve) to get other performance features as Vy. (found where greatest excess power is).
I can´t understand the logic behind that, I mean, why does power affect airplane´s aerodynamic? Why is it important how much power does the engine has if what really makes the aicraft move forward or climb faster is thrust?

Like I said in a comment to the similar question https://aviation.stackexchange.com/q/81809/34686 --"
For a given setting of the throttle or power lever, piston engines with propellers tend to produce constant POWER regardless of airspeed, and jet engines tend to produce constant THRUST regardless of airspeed. ... Of course that is a gross oversimplification. I strongly suspect that this has been covered on ASE before ... search on ASE for other related content

… of course piston engine only produces constant power if it is attached to a good constant-speed propeller so it can be operated at the optimal RPM independent of speed. With fixed propeller at low speed the engine power is limited by not being able to spin up to the optimal RPM and at high speed it has to be throttled down to avoid overreving.

why does power affect airplane´s aerodynamics?

It doesn't. The aerodynamics will determine how much excess power is left for acceleration or climb. But not the other way around.

Why is it important how much power […] the engine has?

Because the power of a piston engine is constant for the same air density, regardless of speed. It is more helpful to give a power rating because that figure is valid over the whole speed range, unlike thrust. Thrust is inversely proportional to speed and so a thrust figure is only valid for exactly one operating point.
Pure turbojets have approximately constant thrust at subsonic speed, so here the thrust rating is more useful. The power a turbojet can produce increases with flight speed.

Definition of Power = Force × Distance/Time = Thrust × Velocity
Another definition Fuel burn/Time.
By the second definition both jet and piston produce "Power", and certainly both jet and piston produce thrust.
We describe prop "power" vs drag because the thrust a fixed prop generates is much more dependent on its forward speed, there for we compare these values to drag at given speeds.
Interestingly, in the real world, we must increase rpm to achieve higher airspeed.  A cleverly designed aircraft would have 2x the rpm at 2x the airspeed, helping maintain the same prop AOA throughout the speed envelope.  This would make a horrid propeller thrust curve much flatter, but there are so many other airspeed/engine combinations to consider.
The jet, spinning at much higher rpm, is much less influenced by forward velocity, so the term is dropped (but could be left in).
The important thing to remember is not to confuse fuel burn power with the "power state" of the aircraft thrust (as Thrust x Velocity).

We only have to increase RPM to achieve higher true airspeed with fixed propeller. And because engine power increases with RPM up to some limit (and then declines quickly and the engine can't handle more anyway), it wouldn't work for large speed range, because then the engine wouldn't be able to produce enough power for take-off. That's why all faster propeller aircraft have variable pitch (constant speed) propellers. And spin them slower at higher airspeed to keep the parasite drag low.

Turbines, i.e. ducted fans, are not influenced by forward velocity because the flow speed through them is almost independent of forward velocity, the inlet converting the speed to pressure (which increases the effective pressure ratio) instead.

@Jan Hudec. "Engine power increases with rpm up to some limit, it won't work for large speed range":  electric motors have a much greater rpm range.  A real opportunity for the application.  Regarding converting speed to pressure, and applying energy to that pressurized air, I am seeing a similarity to the turbojet and fan jet portions (and also to the prop flow stream):  if we increase rho, we can get more thrust (more mass being accelerated).

Electric motor would indeed work with fixed propeller over longer speed range, but ultimately the increasing parasite drag of the faster spinning propeller would become a problem anyway and a variable pitch propeller would be needed for further increase in speed.


text:
Should I keep specific distance from Class B?
I was wondering if I fly my c172 in class E airspace, and Class B base was 5000 MSL. should I keep specific distance from Class B? or is it okay to go up to 4900 MSL but not reaching 5000 MSL?

If you are flying under the Class B shelf, you should be getting flight following and coordinating with ATC on the altitude you are flying at.

There is no minimum (as far as I know) to the distance you should keep when avoiding any type of airspace. However if you are not on top of your altimeter settings at low altitude 100 ft is within an aggressive but potential altimeter setting change and you may accidentally bust the airspace due to an improper local pressure setting.
Generally airspaces (like a class B surrounding a major airport) are made to keep you away from something at the center (major airport) so keeping you a bit further from the edge of the airspace is not really relevant as they are designed to keep you far enough away already.
On the other hand 4900 would be an off beat VFR cruising altitude considering § 91.159 – VFR cruising altitude or flight level.

Some anecdotal advice I heard around the hanger once: ATC is looking at much fancier screens than that iPad in your lap or that 430 on the panel, you should never skirt airspaces that closely no matter the gear you've got


text:
Why did flying boats in the '30s and '40s have a longer range than land based aircraft?
I just learned that until shortly after world war 2, flying boats had a longer range than land based aircraft

In the 1930s, a flying boat route was the only practical means of transatlantic air travel, as land-based aircraft lacked sufficient range for the crossing. (link)

Why was that? I'm not aware of any reason why land based aircraft should have a shorter range than seaplanes.

related: Why were flying boats phased out?

Seaplanes have less air resistance (mostly due to the lack of landing gears that land planes have). That is why most speed records of older times were held by sea planes. That also means more range with same amount of fuel.

@DavidBalažic Seaplanes have a worse aerodynamic shape than land planes because they also need to be boat-like. I'm pretty sure the air resistance of a land plane with retractable landing gear will be less than that of a seaplane, but I'm not sure how seaplanes compare to fixed landing gear aircraft. Do you have sources?

Yes. The speed records. Of course the comparison is to non retractable gears. That is why after they started using retractable gears, the tables turned to favor land planes.

You're right on the speed record thing, but according to Why were the speed records set by seaplanes in the early thirties? this was due to the runway length needed with the fixed-pitch propellers of the day and not so much aerodynamics.

The postwar claim of your question is an erroneous misreading of your link; in actuality that article itself indicates 4000 mile landplane flight between Berlin and NY in 1938.

@ChrisStratton And it also indicates a 5,000-mile NYC-Istanbul seaplane flight in 1931. It further indicates that the first trans-Atlantic passenger flight on an airplane (as opposed to an airship) wasn't until 1939... and it was on a Boeing 314 Flying Boat.

@JanKanis By the late 30s (still before WWII,) constant-speed (i.e. variable-pitch) props were around. The Boeing 314 had them, for example. Also, I don't think it's quite correct to say that it wasn't until shortly after WWII that flying boats had longer range. The B-29 had a longer range than the Boeing 314, for example, and it was definitely around during the war.

@reirab while the wikipedia picture of a CH-300 is of one on floats, every indication I can find is that the NY-Instanbul flight was a land configuration; news articles clearly state that they rolled down the runway at Floyd Bennett, and the picture https://ark.digitalcommonwealth.org/ark:/50959/6682zd329 clearly shows the poise of a taildragger.  So no, that's 5000 mile landplane flight in 1931, not a seaplane one.

@ChrisStratton Ah, fair enough. I didn't look further into it and was just going by the picture there. Certainly seems reasonable enough. After all, Lindbergh flew to NYC-Paris in '27 in a landplane. These were, of course, significantly smaller aircraft than passenger airliners of the day like the Boeing 314 or the DC-3, though.

Since fuel volume in aircraft is a large factor in range, and the volume of an aircraft increases faster than the linear size, larger aircraft have a big advantage for range. In the 1930s, the biggest hurdle for large land-based aircraft was infrastructure. Most airports in the 1930s were not very large, and many parts of the world did not have any at all. The common airliners at the time carried around 20 passengers.
Designing an aircraft that could land on the water allowed aircraft to get larger and have longer ranges without the need for large runways to be built to support them. Aircraft like the Sikorsky S-42 carried almost 40 passengers. It wasn't until at least the late 1930s that land based aircraft were able to reach this size, and after World War II the devolopment of infrastructure and aircraft technology made long-range land-based aircraft much more feasible.

I would also expect it is easier to beef up a fuselage for direct water-landing (as opposed to floats) compared to the weight needed for retractable landing gear. That is, the weight added to make direct water landing reasonable is much less than that needed for retractable landing gear (even if direct water landing requires higher wing mounting or other modification).

@SoronelHaetir I don't have detailed knowledge of aircraft design, but intuitively I don't share that expectation. A flying boat needs to have a strong bottom along the entire airplane to absorb the forces of hitting the water and waves. In a regular plane the skin only needs to withstand aerodynamic forces, and only the relatively small landing gear need to be strong enough to handle the weight of the plane. My intuition thinks the landing gear will be lighter.

@JanKanis If the fuselage can't take the weight of the entire plane plus all its cargo, the plane will collapse. So it already has that strength. The skin itself doesn't have to be thick either, because the force of the water is distributed over a wide area. Wooden sailing dinghies are constructed of plywood as thin as 3mm or 4mm.

@Graham I wasn't talking about weight, but about the impact forces of hitting the water and the waves. A landing flying boat is a lot faster than a sailing dinghy. The weight can be carried by a few structural beams, but all of the underside of the hull must be able to withstand hitting a wave at high speed.

This is why the "Spruce Goose" got built, isn't it - the last attempt at scaling up seaplane technology before the jet age.

Good statement. I had the privilege of growing up in Berlin, which had Hitler's "ORIGINAL" Airport (the first real airport, after the airfields used before) and - it was SMALL. The planes went into Hangars where the terminals where and you walked to the plane IN the hangar. Amazing (at that time only used for mid range turbo prop machines), but it gives a good idea how small the largest aircraft where pre the development of the modern airport and the increase in passenger size and the jet age.Pre this airport basically people used airfields and they were VERY limited.

@JanKanis For the underside of the hull, you're planing - skimming over the water instead of pushing through it. The force from glancing off wave-crests is very different from planting perpendicularly into the water. For sure it needs to be strong, but only on a similar scale to a boat. If you look at Donald Campbell's various speedboats, they were significantly faster than any flying boat, but they weren't that heavily built. Since seaplanes clearly did work, for all the reasons fooot said, I suggest your intuition may not be a totally accurate guide. ;)

@Graham A flying boat requires a large area to be a little stronger, a land based plane requires relatively small landing gear which need to carry the full weight of the plane. I don't know which will be heavier, I guess some numbers would be needed at this point, but I don't have them. Do you have numbers or relevant experience?

I dont think this is worth adding as an answer in its own right, but it does answer some of the questions above - more fuel means more range, which means more weight (as in your answer).  Most airfields pre-WW2 were just that, fields.  Long, muddy fields.  Horrible for heavy aircraft.  WW2 happened and all of a sudden there were an excess of long paved runways pretty much everywhere in the world - these long runways allowed lighter built land based aircraft to carry more weight, meaning more fuel.  Seaplanes were also much more maintenance heavy (sea water being corrosive and all...)

See the progression of London Heathrow from a small unpaved airfield in the 1930s to long paved runways in 1944, with the intention for it to be a hub for long range military transports.  It was the paved runways which made the difference - transports and bombers could carry greater bomb loads further than they could taking off of a grass field.


text:
What is the correct procedure in case of vertical stabilizer failure?
What is the correct procedure in case of vertical stabilizer failure in cruise?
Am I correct in guessing that one should descend to a lower altitude and decrease speed?

Hi M BA, I will edit your question to add a bit of clarity. Next time, please consider being a bit more clear by writing some more details.

I do not understand your query..can you hint at what more I need to add? It is a very unambiguous question to me.

Please have a look at how I edited your question. You'll notice that I have used a bit longer sentences, this helps with clarity. What you could add, for example: what prompted your question (e.g.: are you studying an accident report?), where have you looked for an answer, why is your guess exactly that.

"correct abnormal procedure" what!... should be "what is the correct procedure".

Removal of 'abnormal' and previous edits make this question ok.

I can assure the OP that the aircraft will eventually come to 0 altitude AGL. At what speed is another issue. This question is unanswerable as the specific aircraft design, CG location, and amount of damage will dictate survive-ability. In the case of the USAF B-52H given below, the USAF called Boeing Engineers while it was inflight.

There is no procedure, generally a vertical stabilizer failure makes an airplane uncontrollable, meaning everyone dies. There have been examples where vertical stabilizers failed and people survived, see this question for details. A pilot local to me cut 2/3 of his vertical stabilizer on high tension power wires while scud running and still landed safely.
When there's any control surface failure pilots will have to learn how to fly the airplane and control it using whatever means possible. Differential thrust and speed brakes have been used to control lateral movement (yaw), trim can be used if controls are seized, etc.
Reducing speed may not be a good idea, or at least not all at once, if the vertical stabilizer fails airflow over whatever stump remains may be the only think keeping it straight. If the control is stuck then reducing speed would make sense, it all depends on the nature of the failure. Descending is absolutely a requirement, you have to land somewhere after all, but unless you've depressurized there's no need to rush it, you'd want to work the problem and find out what the new parameters are, then gingerly work your way down.

Most likely, you'd want to retain as much altitude as possible (while retaining enough O2 to live & think) while trying to figure out how to control your crippled plane. More altitude = more time to live.

Well, it all depends @FreeMan, once I figure out how to fly it I'd probably want to get it on the ground sooner rather than later at a facility with the best emergency services and most into wind. The condition of the airplane could deteriorate, and pilot fatigue is a consideration as well. I wouldn't keep altitude just for the sake of it.

@FreeMan: Flying low increases damping, so low and slow is really good advice, especially with swept wing aircraft.

Obviously not a pilot... My thought was that the more room there was between you and the ground the less chance you had of contacting it in an uncontrolled fashion while trying to figure out how to control what's left of your aircraft. I yield to those with much greater knowledge who indicated my assumptions were incorrect.

That's a common thought @FreeMan, and it is right in some situations but not all of them.

Am I correct in guessing that one should descend to a lower altitude and decrease speed?

Yes, if you fly an aircraft with a backward swept wing. Otherways, losing the fin will be impossible to compensate.
This answer explains how a backward swept wing will aid in directional stability, and this help increases with angle of attack. Therefore, flying slow is the most important step in order to restore directional stability. Flying low increases aerodynamic damping, so low and slow is the best choice in this case.
@Carlo mentions B-52H 61-023 which lost most of its vertical on January  10, 1964. This aircraft survived, but a number of them who suffered the same fate crashed. There were several factors which helped:

Just enough of the tail was left to keep the aircraft flying. Later simulations showed that a full loss of the vertical would had been impossible to compensate.
The crew lowered the rear landing gear, so the gear doors helped by doubling as ventral fins. Every bit counted on that occasion!
Also, the loss of mass from the lost tail portion shifted the center of gravity forward, which is another way to improve directional stability. The crew did all they could to get the center of gravity forward, and again, without that the aircraft would had lost directional stability.

What is the correct abnormal procedure in case of vertical stabilizer failure in cruise?

My advice: Don't count on being as lucky as B-52H 61-023, but bail out as long as you can!

Depends on the aircraft.  Generally structural failure results in departure from controlled flight, though there are a few exceptions such as this USAF B-52H which lost its vertical fin over Colorado on Jan 10, 1964.

Oddly enough, the loss of a vertical fin is probably the least riskiest form of structural failure.  An airplane can still maintain approximate directional control from the longitudinal center of pressure from the tailboom of the airplane well behind the center of mass, provided all other systems remain functional.  The airplane can still be turned using aileron input, though slipping and skidding from adverse yaw and other factors are going to be much more pronounced demanding greater care in handling at lower speeds.  Multiengine aircraft will most likely depart from controlled flight here in the event of an engine failure.  Again the severity of the loss of control here will depend on the amount of structure lost and it’s effect on directional control.

For this type of failure, the B-52 has an advantage in that it doesn't have ailerons.  It relies entirely on spoilers for roll control. The spoilers deploy on the wing in the direction of the turn, adding drag as well as reducing lift. This generates the roll with almost no adverse yaw.

Even with a little adverse yaw, it does not necessarily mean a loss of control, you still have quite a bit of surface area from the tailboom to provide some directional stability, though care should be taken - never try something risky in a compromised airplane.


text:
How do I determine the maximum thrust that a Tesla powertrain (P100D for example) could produce if attached to a single propeller?
I am theorizing a drone helicopter from a tesla powertrain (P100D).
Lets assume the system is theoretically maximized with an optimum propeller (this is what I am currently stuck on).  How much thrust could a Tesla powertrain produce (I understand the thrust would decrease as the battery drains and voltage decreases, just wanting to get an idea of theoretical optimal thrust)?

Try something. Put your car's accelerator pedal to the floor and hold it there continuously for an hour. You can't. Within 10 seconds, safe operation will oblige you to abort the experiment. Car engine design absolutely relies on that; they're like a starter motor, good for 5 seconds but absolutely not made to run that power continuously. Your "200hp" engine might be good for 30hp continuous. Tesla's powertrains will have the same constraint, the motor isn't sized for continuous at-rating operation.

For a propeller/rotor, net consumed power = torque * rpm. For an electric drivetrain, net power delivered = current * voltage, with current equating to torque and voltage to rpm. Tesla should have really good data analyses on the power delivery as a function of demand over time, with wheel torque and wheel rpm as parameters.
Then you only have to replace the propellant medium: slippery air instead of the stony fixed world. Torque is a measure of drag of the rotor blade, thrust is a measure of lift. Both are a function of rpm as well, usually the rpm is limited by the tip speed approaching the speed os sound and is chosen to be as close as possible to it.
So a design process might be:

Determine payload.
Determine approximate disk load by comparing with existing designs, resulting in the disk area and the blade radius.
Compute rpm so that tip speed is about Mach 0.8
Compute power, torque and thrust from data available for comparable comfigurations.

Two books that contain chapters on pre-design of helicopters are Helicopter Performance, Stability and Control by Raymond Prouty, and Helicopter Aerodynamics by J. Gordon Leishman.
But the area that will give you most headache won't be computing the thrust etc, it will be figuring out how to control the flight path. Do you plan to use a swash plate?


text:
How are empennage airfoils chosen?
Empennage airfoils are usually symmetric. What is the process behind choosing the airfoil used in the tail of the airplane? Is it related to the stability? And then how would the angle of attack of the tail be determined?

Selection of the profile is only a small portion of the design procedure of the empennage:

Aerodynamic centre of the aircraft minus tail is determined first.
Aft Centre of Gravity is determined.
Static longitudinal stability criteria determine the area of the empennage - upon a disturbance in Angle of Attack, the total aircraft must generate a moment that counteracts the disturbance. Usually, wings and fuselage contribute negative stability, and the tailplane must be sized such that the stabilising moment compensates for this in all circumstances.
Once the area is determined, the aspect ratio, taper ratio, sweep angle and profile thickness are chosen considering a host of factors, including the lift-curve slope, stall behaviour, out-of-trim conditions, structural weight, design diving Mach number.
And then the airfoil shape is chosen. From Torenbeek section 5.5.1.d:

The basic requirements are that the airfoil section should have a high $C_{L_\alpha}$ and a large range of usable angles of attack. Frequent use is made of approximately symmetrical airfoils with a thickness ratio of 9 to 12 percent and a large nose radius, e.g. NACA 0012. Tailplane stalling - at
  the lower surface - can be postponed by adopting negative camber (e.g. NACA 23012 upside down), upward nose droop, an increase in the nose radius or by means of a fixed slot.

Table 9-2 from the same book contains (dated) horizontal tailplane design data and lists some profiles used:

Cessna 177: NACA 0012/009
Fokker F27: NACA 63A-014 mod.
Lockheed C-130B: NACA 23012 inv.
Cessna Citation 500: NACA 0010/0008
B737/100: 12%/9%
Boeing 707/320: BAC-317

To answer your three questions one by one:

What is the process behind choosing the airfoil used in the tail of
  the airplane?

Initially, some generic airfoil is chosen during development and the size is determined from the tail volume of comparable designs. Next, all load cases are determined and checked against the preliminary tail. Its size is adjusted until all requirements can be covered. Normally, the required lift coefficients are not spread out symmetrically, so the camber and incidence are adjusted such that drag in cruise is minimal. This in combination with the powerful flaps explains why most airliners have a negatively cambered tail. In a last step, the tail airfoil can be optimized such that the same tail load can be generated with a smaller tail surface. Or the designers stick with what they know.

Is it related to the stability?

Yes, but stability drives tail size, not the choice of airfoil. More stability means a more forward c.g. location and more variation in the load at the tail between slow and fast flight.

And then how would the angle of attack of the tail be determined?

This can be done by a simple potential flow analysis. This answer explains step by step which effects add up to determine tail angle of attack. And then some aircraft prefer to set the incidence of the tail in flight, so the angle of attack (more precisely, the combination of angle of attack and elevator angle) can be adjusted.


text:
How are flaps sized in order to give necessary delta CLmax?
Given an airfoil and wing planform, what is the process to find the flap size and deflection necessary to give a certain change in CLmax? Flaps are usually sized as a percentage of the chord, with 0.2c to 0.3c being the normal range for most aircraft. But how do you find the length of the flaps spanwise?

That answer did not cover the analytical method of determining CL change. I'm unsure what this iterative method is. Is the only method using CFD or wind tunnel testing? How were they sized before these technologies were available?

This is to answer your follow-on question in the comment;

That answer did not cover the analytical method of determining CL change. I'm unsure what this iterative method is. Is the only method using CFD or wind tunnel testing? How were they sized before these technologies were available?

I feel the main question has been answered already here.
Flaps, especially the ones with gaps between the wing and the flap, depend a lot on how the boundary layer forms and when separation occurs. Therefore, CFD is still struggling to get everything right and wind tunnel testing is still needed at least to verify the computational simulation.
Before CFD, the average aircraft engineer would experience a number of designs in his (rarely her) lifetime which gave him a feeling what works and what not. Still, surprises were not uncommon and late fixes were needed to arrive at the desired performance.
If you desire to use a simple formula which precisely predicts the maximum lift coefficient, including size, speed, temperature, airfoil shape and manufacturing imprecisions: It does not exist.


text:
What are the conditions for an airfoil to be a "thin airfoil?"
I know about the theory differences between regular airfoils and thin airfoils, but is there any condition for saying a given airfoil can be analyzed as if it is thin? The extreme case of infinitely thin makes sense, but I am curious if there is some cutoff dimension that determines thin or not thin.

There is no hard boundary. Normally it is around 8% relative thickness, details depending on camber and nose shape.
Typical for a thin airfoil is a stall originating from the nose, with a sudden separation of upper side flow, while thicker airfoils start to stall with a separation starting from the trailing edge and moving gradually forward. This gives thin airfoils a nasty stall behavior while thick airfoils stall in more benign ways. The stall behavior depends not only on thickness but also on camber and the geometry details of the airfoil nose, but can be used to separate thin from thick airfoils.
Thin airfoils make sense in two applications:

when the local angle of attack is well controlled, such as in flaps and turbo machinery, and
for trans- and supersonic flight where thickness causes wave drag.

In all other applications thicker airfoils with a blunter nose should be preferred because they allow to store more fuel and to make the load-carrying structure more efficient. The upper limit of practical airfoils is at 20% to 22%, the root thickness of the Davis wing as used in the B-24 and B-29.

Airfoils are usually divided in "thin" and "thick" according to their stall behaviour: trailing edge stall, leading edge stall, and thin airfoil stall. One of the main defining parameters in how the wing stalls is the thickness of the wing profile.
From Torenbeek, both the pictures and the citations:

Trailing edge stall

This type of stall is characteristic of most airfoil sections with thickness/chord ratios of approximately 15% and above. The flow at large angles of attack is characterized by a progressive thickening of the turbulent boundary layer on the upper surface. As the angle of attack is increased to about 10 degrees (B), flow separation starts at the trailing edge and moves gradually forward.

Leading edge stall

Airfoils with thickness/chord ratios of about 9 to 12 percent experience an abrupt separation of the flow near the leading edge. On these sections separation of the laminar boundary layer occur well before the attainment of maximum lift and prior to transition to a turbulent boundary layer. Transition occurs in the shear layer thus formed, and the expansion of the turbulent motion spreads at such an angle that re-attachment of the flow quickly occurs, enclosing a "short bubble" and subsequently forming a turbulent boundary layer (B).
The lift and pitching moment curves exhibit abrupt changes when the angle of attack for maximum lift is exceeded. There is little or no rounding of the lift curve and a sudden negative shift of the pitching moment resulting from the rearward shift of the centre of pressure is observed.

Thin Airfoil stall

On very thin sections of thickness/chord ratios of less than about 6 percent and on round noses a small separation bubble occurs at very small angles of attack (S). At a certain critical angle of attack the short bubble breaks down, but the flow subsequently re-attaches downstream, forming a "long bubble" which causes a slight reduction in the lift-curve slope (B). With increasing angle of attack the point of flow reattachment progressively moves backward until it coincides with the trailing edge and maximum lift is reached at this condition (C)

So according to the categorisation of airfoils in three types of stall, "thin airfoil" means a chord thickness of 6% or less.

As much as I respect Torenbeek, his distinguishing of three types of stall is a bit arbitrary. After all, his second case is just like the first, only that in thinner airfoils the laminar separation bubble rushes forward more quickly with increasing angle of attack. And his "thin airfoil stall" can be observed with airfoils of 10% thickness as well if they have little camber and a small leading edge radius.

Yes the mechanics are pretty much identical, it's just the stall behaviour that differs, the leading edge stall being very nasty with the sudden drop in $C_L$, the thin airfoil $C_L$ drop being much more benign. Torenbeek does state in the book that there are many more variables, indeed such as the ones you mention. Thanks for the edit by the way.


text:
There's an airfield somewhere in the United States with a metal runway. Where is it?
Somewhere in the United States, there's an airfield (probably military) with a runway made entirely out of metal.  I've seen the approach plate for it, but I can't remember the identifier.
What airfield am I thinking of?

You are thinking of Bogue Field (KNJM). It is a Marine Corps auxiliary landing field, currently mocked up to look like a carrier deck, and last I heard it was still active and in use to practice carrier landings.
The runway surface is Perforated Steel Planking (PSP - actually I believe the new stuff isn't "steel" anymore but the name remains) - discrete sections that can be laid out to quickly construct temporary landing strips in forward combat zones.

A search on AirNav for "pierced steel plaking" came up with a couple more military fields:
KNXP   Twentynine Palms SELF Airport (Strategic Expeditionary Landing Field) -
Twentynine Palms, California, USA
2CA9   Castle Dome Army Heliport -
Yuma Proving Ground (Yuma), Arizona, USA
2CA9 says it's a helipad but it's 3000x65. The rest of the results were all (reasonably sized) helipads.

if 2CA9 exists, i couldn't find it on gmaps

you think 2000x72 is reasonable size? "CAUTION: HELICOPTER LZ (HLZ) WHITE RHINO (UAS STRIP 2000' X 72') S OF AND PARALLEL TO RWY 28 IS PRIM FOR TENANT UAS SUPPORT, RSTD TO UAS AND COPTER OPS ONLY"


text:
What's a CWS button?
I've flown a Piper that has a button labeled CWS on the yoke, right next to the "Push-To-Talk" button.  I never got a chance to push it though.  What does it do?

"I never got a chance to push it though"; I take it you're not familiar with the famous last words "what does this button do?" :)

Sounds like that plane has (or once had) an STEC autopilot installed - CWS stands for Control Wheel Steering - basically it allows you to set a pitch attitude / turn rate using the yoke, which the autopilot will then maintain until instructed to do otherwise.
If the autopilot is still installed you also normally use the CWS mode as part of the preflight routine to get the autopilot to activate so you can make sure you can overpower it (details on the preflight as well as using the CWS feature can be found in the autopilot manual, which should be with the POH).
If the autopilot is no longer installed the switch technically should have been removed with the rest of the system and a blanking plate installed in its place.

Huh. Now I wish I had a chance to try that out.

CWS is Control Wheel Steering, an early autopilot feature that senses if the pilot presses the flying control, and adjusts the target pitch as a result. Forces over a certain threshold activated the CWS when selected, however the required forces are much below the autopilot override force, so the autopilot stays fully functional.
It is still present in the B737-NG autopilot select panel, the MCP. CWS-R activation played a roll in the crash of Flash Airlines flight 604.


text:
What (if any) endorsement do I need to give a student who scored 100% on his knowledge test?
My student studied his empennage off and got a 100% on his knowledge test.  Usually, when I endorse students, there's a sentence that goes:

I have given [name] X hours of instruction on the areas in which he/she was found to be deficient on his/her airman knowledge test.

In this case, my student wasn't deficient on any areas of the knowledge test.  Do I need to make that endorsement?

If we're talking about FAA regs I'm not aware of any endorsement required if your student passes the test with a perfect score (they need an endorsement to take the test - I have given the instruction required by..., and they need an endorsement like the one in your question if they fail or don't manage a perfect score).
The endorsement above is intended for use when you need to endorse someone who failed to re-take the exam (similar to what's done if they fail a checkride), or when there's stuff to review from the knowledge test (My instructor combined it in my Checkride endorsement - And has demonstrated satisfactory knowledge of the areas found deficient on the knowledge test.
That said, it is probably a good idea to cover anything the student missed on the exam or doesn't feel 100% confident on, and to log that training - in this case if you do additional ground work you would just log it as you normally do...

@CGCampbell Massively rewritten - do not post answers while at conventions :-P

Also, as usual, There's an AC for this (AC 61-65E)

I obtained 100% on my Private Pilot Airplane Single Engine Land (ASEL) test in the US. My instructor didn't need to provide an additional endorsement.
However, upon seeing my score, the Designated Pilot Examiner (DPE) was especially tough. After a hard grilling, she admitted to me: "I wanted to go hard on you because 100% on your FAA test doesn't mean you know everything you need to be a safe pilot. The license you just got is a license to learn, with flying privileges - don't ever forget that."
I have taken that lesson home.
When I took the IFR FAA test, I purposely missed a question on a subject I was particularly strong in and got a 98%. When the DPE (a different one) asked me questions on the subject, he was pleased I understood the concepts very well.


text:
What can I do on the ground in order to help my body acclimate to aerobatics?
I've just started aerobatics training!  Unfortunately, my middle ear really messes with me, particularly during loops. Strangely, aileron rolls give me no trouble at all.
I know from other experienced aerobatic pilots that you can get used to aerobatic flying, and my tolerance will build up over time.
However, is there anything I can do on the ground in order to help build up my tolerance more quickly?

As you've mentioned, your tolerance will build up over time so a simple, partial answer is to be patient and fly in small doses. Two 20 minute flights with a break between is much much better than a single hour long flight.  You'll learn more, stay focused, and acclimate faster.
Try not to move your head much during maneuvers.
For a loop, I typically look at my left wing tip until I'm about 3/8's through the loop, then I transition to looking "up" to catch the horizon as I approach the top of the loop. Any more movement than that might be a problem.
Bill Thomas's classic book Fly For Fun describes where to look for many of the basic maneuvers.

It's cool there are people who know this sort of thing  :)

The way the Air Force introduces people to the disorienting effects of aerobatics on the ground is to put them into smoothly spinning chairs called Barany Chairs.  And it doesn't just involve spinning in one direction, but smoothly and gently changing directions repeatedly.  Since I never suffered from airsickness, I don't know what they do (i.e. do they blindfold you, are there more than one axis of motion, do you have to tilt and move your head, etc.)
While an actual Barany Chair may be expensive, you could perhaps accomplish the same with  a friend and a nice heavy office chair.

the mythbusters once made a seasickness (or general motion sickness chair) that rotated steadily and the subject had to tilt his head back and forth and side to side.

I'll give this a try and report back with my results!

Given that I'm sitting on an office chair at the moment, I have a temptation to spin myself right now

The Mythbusters chair is a good starting point - it won't get you inverted, but much like a Bárány chair it can be used to habituate your body to some of the sensations you'll experience. Beyond that it's just a matter of flight time (and it's best if you stop before you get sick).

I used to feel strong motion sickness when flying aerobatics. I got to the point I could burn off the acro tanks in an Extra 300 without getting sick. I did 3 things:
1) Repeatedly spinning in an office chair until I felt dizzy then getting up and walking.
2) Flying as much as I could, and when I did, I would fly until I felt sick, then fly straight and level until I felt a bit better, then would do an avalanche maneuver (snap-roll at the top of a loop), then would fly straight and level again... and do it again, gradually pushing myself.
3) If you're in a car and not driving, try reading until you start feeling sick.
Somehow I never threw up flying aerobatics, but I did come really close a number of times.
I tried to relax, control my breathing, bring a little water with me, and always push myself a bit. And somehow that worked for me. Also if you do some aggressive maneuvers like lomcováks, long tail slides, push humpty-bumps, and accelerated spins, most other maneuvers will seem much more tame and you'll be able to relax and enjoy them.
Good luck and have fun!